Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BACKGROUND OF THE INVENTION
This invention relates to a viewing and measuring apparatus~
and specifically to an apparatus wherein the measuring point is sub-
stantially coincident with the viewing point.
Simple comparators are known in which a measuring scale or
grid is superimposed with the object. These devices use relatively
coarse scales, and attempting to increase precision and accuracy
further by finer division of the scale results in an obscured view of
the object. Such a device is not adaptable to the high precision
automated measuring instruments to which the present invention is
addressed.
Precise and accurate optical measuring devices, per se, are
known. For example, United States Patent No. 3,996,463, to Jablonow-
ski, describes the use of grating interferometry to monitor relative
movement between a beam of coherent light and a grating. U.S, Patent
No. 3,904,401 ~o Hock et al, describes an interferometric measuring
system without systematic markings on ehe measuring surface. However,
neither system provides for viewing of an ob~ect to be measured.
Moreover, as will be described, attempeing to add viewing capabilities
to such a system presents a number of difficulties, particularly if
the object is to be superposed with the grating.
Optical measuring systems are known which provide a view of
the object being measured. United States Patent 3,729,830, to Blachut
et al, uses encoding devices that are offset from the viewing ones to
allow a clear view of the object. A viewing mark or dot provided by
the viewing optics represents the points on the object to which coord-
inate measurements are related. The accuracy of measurement is
limited by the stability of the mechanical linkage between the viewing
system and the measuring system, when translated, due, for example, to
bearing freeplay and runout, and deformation of linkage elements.
In attempting to provide suitable viewing capabilities for
~.
an interferometric system without loss of measurement accuracy, a nu~-
ber of difficulties are encountered. As indicated above, the conven
tional approach of keeping the viewing and measuring systems physically
separate, has accuracy limits due to mechanical guidance errors which
are very difficult to improve further. On the other hand, physical
superpositioning of the object and grating produces undesirable visible
effects in the viewing system. Although grating lines per se, can be
made invisible in the viewing system, leakage of the measurement bea~
into the viewing channel produces distracting pulsing of the beam
intensity in the viewing system when translating the object-grating
unit. Also, the conventional approach for multiplexing an optical sig-
nal for independent utilization, e.g~ viewing and measuring, involves
the use of broadband beamsplitters which reduces the signal by at least
50% for each pass, significantly reducing optical efficiency.
Another approach for multiplexing is using frequency-sepa- ;
rated carriers (coloured channels) coupled and uncoupled by dichoic
beamsplitters, This provides improved efflciency for one channel but
imposes chromatic restrictions. The performancs is further degraded if
one of the channels is broadband as is typical for a viewing system
which the present invention incorporates.
SUMMARY OF T~E INVENTION
An object of the present lnvention is to provide a viewing
and measuring apparatus wherein the measuring point is substantially
coincident with the viewing point.
Another object of the present invention is to provide a
viewing and measurement apparatus wherein the viewing and measurement
spectra are efficiently multiplexed along a common optical axis.
The present invention comprises a viewing system having an
optical viewing axis which intersects the object at a viewing point;
an interferometric measuring æystem having an optical measuring axis
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which intersects the object at a measuring point; optical modulating
means superposed with the object, said modulating means providing
relatively high spacial frequency content matching that of the
measuring system; reflective means superposed with said object for
providing at least partial reflection of incident sp~ctra; means for
supporting the viewing system and the measuring system relative to one
another such tha~ the viewing system axis and measuring system axis
intersect the object substantially coaxially and normally, whereby the
viewlng point and measuring point are substantially coincident; means
providing that a spacial frequency plane of the viewing system is sub- -
stantially optically coplanar with a spacial frequency plane of the
measuring system to define a common spacial frequency plane, at wh~ch
plane relatively high frequency spectra operatively associated with
measurement are spacially separated outwardly from relatively low
frequency spectra operatively associated with viewing; reflecting
means optically associated with the spacial frequency plane for selec-
tively deflecting the high and the low frequency spectra from each
other to effect axlal separation thereof and, whereby the separated
low frequency spectra can be utilized in said viewing system; said
measuring system including photo detection means fixed relatlve to the
viewing-measuring axis for receiving separated high frequency spectra
reflected from the surface of the grating; and, means for translating
the object relative to the viewing-measuring axis within the ob~ect
plane to effect visual translation of the object in the viewing system
while interferometrically measuring the translation.
BRIEF ~ESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic representation of one embodiment of
the present invention.
Figure 2 is an enlarged perspective representation of the
aperture plate and detector assembly shown in figure 1
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Figure 3 is a schematic representation of an alternate
embodiment of the present invention.
Figure 4 is a cross-sectional enlarged view of a preferred
embodiment of an object-grating unit.
DESCRIPTION OF PREFERRED EMBODIMENTS
Figure 1 illustrates one embodi~ent of an apparatus for
viewing and measuring an object 1. Superposed with the object 1 is a
grating 2 with a partially reflective grating surface, such that the
object and the grating are substantially in the same plane 3. The
superposed object 1 and grating 2 together define an ob~ect-grating
unit 4~ The object-grating unit 4 is shown supported on supporting
structure 5 adapted to be translated laterally wi~hin the object plane
3.
Preferably, the object 1 and grating 2 will be arranged such
~hat the grating surface of the grating 2 is in proximity with the
1 surface of the object 1 being examined, in order to minimize the
superposition error, i.e., the separatiQn due to the grating and
ob;ect thickness
The apparatus of figure 1 includes a viewing system and a
measuring system.
The viewing system as shown comprises a transparent ob~ect
1, an objective lens 6, reflecting means 7, imaging lens 8,
illumination source 9, and image plane 10. The optical axis 11a of
the viewing system intersects the object 1 at a viewing point 12a~ A
viewing mark 13 centered in the image plane 10 represents the viewing
point 12a to a viewer. It will be understood that the viewing system
components shown may form the front-end of a more comple~ image
processing and/or viewing system.
The measuring system of the present invention incorporates
an interEerometer, the operating principle of which is known in the
art, (see, for example, the aforesaid Jablonowski patent).
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The measuring system as shown in figure 1 comprises a beam
source 14, an aperture plate 15, the objective lens 6~ the Brating 2
with partially reflective surface, and photo-detector means 16, all of
which are aligned along an optical axis 1lb, which intersects the ob-
ject at a point 12b, herein referred to as the measuring point. The
beam source 14 provides multiple beams 18 that are made to interfere
at the grating 2. Figure 2 shows the aperture plate passing four
beams, three or more being required for a two-dimensional system. The
component beams are substantially monochromatic and have a specific
spacial orientation, phase and polarization relationship dependent on
system configuration. ~eam sources may employ masked, expanded laser
beams, as in the aforesaid Jablonowski patent9 or diffraction orders
emitted from gratings, as in the Hock et al patent.
The components of the viewing and measuring system are sup-
ported reIative to one another such that the viewing point 12a and
1 measurin~ point 12b are made substantially coincident, and the viewing
system axis 11a and measuring system axis 11b are suhstantially coax-
ial. It should be noted that figure 1 shows the axes 11a and 11b, and
points 12a and 12b, displaced from one an~ther only to facilitate
representation~
Since the object and grating are substantialy in the same
plane, both the viewing system and measuring system have a common
spacial domain at the obJect plane 3. By virtue of the shared optical
element, the objective lens 6, both systems transform into a common
spacial frequency domain at plans 17b. For the viewing system, the
spacial frequency domain also exists at plane 17a. The planes 17a and
17b are rendered optically equivalent by placing the reflecting means
7 in the bisecting plane 17c. In this manner, the spacial frequency
planes of the two systems are made optically coplanar.
In the spacial frequency plane 17b, different spacial
frequencies are separated spacially from one anothsr with the
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relatively high frequency spectra, associated with measurement~ being
located outward from the relatively low frequency spectra associated
with viewing. This separation exists when the grating lines are at
least twice as fine as the smallest feature to be resolved in the
viewing system.
To maintain separation of the low frequency spectra from the
high for independent utili~ation, reflective means 7 is arran8ed to
deflect the low frequency spectra from the axis of high, at the spac-
ial frequency plane 17b. The axially separated, low frequency spectra
~ 10 can then be utiliæed, e,g. viewed at 10, free from the modulation
; effects of the measurement spectra~ It should be noted that separa-
- eiOn is achieved with high optical efficiency when compared with the
use of either broadband or dichroic beamsplitters for this purpose.
Using a beamsplitter results in at least 50X loss of signal for view-
`~ ing and/or measuremen~, while the present invention incurs no signifi-
-~ cant loss of signal at this stage for either viewing or measuring.
In the measuring system, beams 18 from source 14 are passed
through apertures 19 in the aperture plate 15, which lies in the
spacial equency plane 17b. The beams 18 are Fourier transformed
into the spacial doma~n plane 3 at the grating 2 by the obJective lens
6. The reflected beams, reflected from the partially reElective sur-
face of the grating, are then transformed again, by lens 6, into a
con~ugate frequency domain at plane 17b for detection by photodetec-
tors 16, which are also located in plane 17b. As can be seen in
figure 2, the photodetectors can be conveniently mounted in the aper- ;
ture plate 15. Associated with the detectors are suitable processing
means 20.
The measurement system components are configured such that
there is a characteristic system spacial frequency upon which correla-
tion is made in the object plane. This characteristic frequency must
be present in the object plane and is most effectlvely provided by a
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matched grating. Alternatively, as disclosed in the aforesaid Hock
patent, the ambient spacial frequency content of an object may be uti-
lized to function as the grating element. Although the signal will be
less than that of a conventional grating, it may be adequate for cer-
tain applications, such as where large objects are involved. The
measurement technique, per se, is known in the art and involves detec-
tion and counting the cycles of variation in light intensity reflected
from the reflective object surface as it is translated within the
plane 3.
The grating may take various forms, and will preferably be
of the type optimized to reflect the selected diffraction orders.
Figure 4 shows the preferred construction of a grating 31 in relation
to an object 35. The grating substrate 31 has a coating 33 of reflec-
tive material on the grating surface 32 designed to provide partial
reflection and partial transmission. Over the coating 33 is a layer
34 of material having a refractive index matching that of the grating
1~ .
substrate, adapted to provide a planar outer surface for superposing
with the object 35. This composite structure performs as a typical
gratin~ in reflected light, but it prevents the formation of transmit-
ted side orders since the grating effect of periodic phase moaulation
is nulled by the layer 34. The absence of replicated transmission
spectra permits utilization of a wider bandwidth of viewing spectra,
with spacial frequencies approaching that or the grating.
The embodiment shown in figure 1 is arranged for transparent
objects, such as photographic transparencies. The embodiment of fig-
ure 3 may be used for either transparent or non-transparent objects.
For non-transparent objects, the viewing system comprises
illumination source 21, beamsplitter 22, objective lens 23, and
imaging lens 24 for viewing the object 25 at 26. Figure 3 shows an
annular deflecter 27 that deflects the outer high frequency spectra to
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be detected by photo decectors 28, while the low frequency spectra
passes through thP central opening 29.
In the case of transparent objects, illumination source 30
may be used, and illumination source 21 and beamsplitter 22 are then
not rPquired.
The object and grating may take various forms. For exampla,
the grating may be combined with the object to form an integrated unit
similar to that illustrated in figure 1, or, as in figure 3, the
grating may be incorporated into a supporting element on which the
ob~ect is placed. Also, as already described, the ambient spacial
frequency content of the object itself can be utilized, eliminating
the requirement for a discrete grating element. Similarly, tha ambi-
; ent reflectivity of the object may be utilizedJ eliminating the
requirement for a separate reflective layer.
It will be understood that viewing will not necessarily be
done by an operator but may, for example, be per~ormed by a suitable
~ .
photo-receptive devlce. In such application, the viewing mark would
be omitted. It will also be apparent that the viewing optics may be
used in reverse, to generate a pattern on a photo-sensitive object,
for example. In any application, the present lnvent~on will provide
separation of measurement spectra from the "viewing' syste~.
It will, also, be apparent that the position of the viewing
illumination and imaging optlcs can be reversed, by introducing an
additional objective lens into the viewing system. In either case>
the spectra of each system are efficiently multiplexed since an unmod-
ulated illumination source has a spacial frequency of zero.
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